Twin-clutch transmission system

ABSTRACT

A twin-clutch transmission system in a power unit for a vehicle includes a clutch system with a pair of hydraulic clutches, which respectively include clutch pistons. A gear transmission includes a first main shaft, which has a first end portion linked to a first one of the two hydraulic clutches. The gear transmission also includes a second main shaft, which has a first end portion linked with the other one of the two hydraulic clutches. A pair of hydraulic passages are concentrically formed inside the second main shaft, and apply hydraulic pressure respectively to the clutch pistons. An operating fluid is supplied from a second end side of the second main shaft. The resulting configuration enables the protrusion of the power unit toward the clutch system to be minimized.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2006-233987, filed Aug. 30, 2006, the entirecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power unit for a vehicle in which thedriving power is transmitted from an end portion of a crankshaft, via aprimary reduction system and a clutch system, to a gear transmissionhoused in a crankcase of an engine. The invention particularly relatesto an improvement on a twin-clutch transmission.

2. Description of Background Art

Japanese Patent Application Laid-Open Publication No. 2003-320861discloses an example of power units for motorcycles in which a leverswitching the clutch system between the engaged state and the disengagedstate is placed outside of a clutch cover, which covers the clutchsystem.

The structure of a clutch system with a twin-clutch structure composedof a pair of hydraulic clutches requires an arrangement such that arelatively large part of the clutch system protrudes from a side of thecrankcase along the axial direction of the crankshaft. When, as JapanesePatent Application Laid-Open Publication No. 2003-320861 discloses, theactuator switching the clutch system between engagement anddisengagement is arranged further outside of the clutch system, theactuator, in such a structure, is arranged further outside of the clutchsystem that protrudes, to relatively large extent, from the crankcase.Such a structure leads, in the case of a motorcycle, to a smaller bankangle, and makes the space for the rider to rest his/her foot on hard tobe obtained.

The present invention is made in view of the above-described problems.An object of the invention is to provide a clutch transmission in apower unit for vehicles with the following characteristics. While theclutch system has a twin-clutch structure, the power unit has as littleprotrusion toward the clutch system as possible.

SUMMARY AND OBJECTS OF THE INVENTION

To accomplish the above-described object, a first aspect of the presentinvention provides a power unit for a vehicle, in which the power istransmitted from an end portion of a crankshaft to a gear transmissionhoused in a crankcase of an engine, via a primary reduction system and aclutch system. In the power unit, the clutch system includes a pair ofhydraulic clutches respectively having clutch pistons. The geartransmission includes a first main shaft, which has a first end portionlinked with a first one of the hydraulic clutches, and which is placedin parallel to the crankshaft, and also includes a second main shaft,which has a first end portion linked with a second one of the twohydraulic clutches, and which coaxially penetrates the first main shaft.A pair of hydraulic passages are concentrically formed inside the secondmain shaft to apply hydraulic pressure respectively to clutch pistons ofthe hydraulic clutches as allowing the operating fluid to be suppliedfrom the second end side of the second main shaft.

A second aspect of the invention includes a hydraulic control meanswhich controls the supply of operating fluid to the two hydraulicpassages, and which is placed on an opposite side-surface of the enginefrom the clutch system.

A third aspect of the invention additionally includes an engine coverjoined with the crankcase, and a support plate attached to the enginecover and supporting the hydraulic control means. A pair ofcommunicating-fluid passages are provided to link the hydraulic controlmeans to the two hydraulic passages. A part of at least one of thecommunicating-fluid passages is formed between the attaching surfacesrespectively of the engine cover and the support plate.

Effects of the Invention include the following:

In the first aspect of the invention, the clutch system including a pairof hydraulic clutches, and the gear transmission including the coaxiallyarranged first and the second main shafts, each of which has a first endlinked with the corresponding one of the two hydraulic clutches,constitute a twin-clutch transmission. Inside the second main shaft,which is coaxially surrounded by the first main shaft, a pair ofhydraulic passages are concentrically formed to apply hydraulic pressureto the clutch pistons respectively of the two hydraulic clutches. Theoperating fluid is supplied to the hydraulic passages from the secondend side of the second main shaft. Accordingly, a large sidewardprotrusion of the power unit can be avoided even on the side where theclutch system is located. This, in turn, allows a larger bank angle tobe set when the power unit is mounted on a motorcycle.

According to the second aspect of the invention, the crankcase isprevented from having a larger vertical dimension around thetransmission housing, which is formed inside the crankcase toaccommodate the gear transmission therein. Thus a smaller crankcase isaccomplished.

According to the third aspect of the invention, a part of at least oneof the communicating-fluid passages, which link the hydraulic controlmeans with the two hydraulic passages, is formed with ease.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a simplified diagram showing a basic configuration of a powerunit;

FIG. 2 is a vertical cross-sectional view showing a part of atransmission system;

FIG. 3 is an enlarged view of the right half of FIG. 2;

FIG. 4 is an enlarged view of the left half of FIG. 2;

FIG. 5 is a view on the arrow 5-5 of FIG. 2;

FIG. 6 is a cross-sectional view taken along the line 6-6 of FIG. 3;

FIG. 7 is a chart showing the change in revolution rate of a first and asecond main shaft at the time of down-shifting while the vehicle runs insecond speed;

FIG. 8 is a diagram showing a route that the power is transmitted whensynchronizing means operates at the time of down-shifting while thevehicle runs in second speed;

FIG. 9 is a chart showing the change in revolution rate of the first andthe second main shafts at the time of up-shifting while the vehicle runsin second speed; and

FIG. 10 is a diagram showing a route that the power is transmitted whenthe synchronizing means operates at the time of up-shifting while thevehicle runs in second speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 10 shows an embodiment of the invention. FIG. 1 is asimplified diagram showing the basic configuration of a power unit. FIG.2 is a vertical cross-sectional view showing a part of a transmission.FIG. 3 is an enlarged view of the right-hand half of FIG. 2. FIG. 4 isan enlarged view of the left-hand half of FIG. 2. FIG. 5 is a view inthe direction of the arrow 5-5 in FIG. 2. FIG. 6 is a cross-sectionalview taken along the line 6-6 in FIG. 3. FIG. 7 is a graph showing thechange in the revolution speeds of a first and a second main shafts,which change takes place at the time of down-shifting while the vehicleruns in second gear. FIG. 8 is a diagram showing a power-transmissionroute at the time when synchronizing means operates at the down-shiftingwhile the vehicle runs in second gear. FIG. 9 is a graph showing thechange in the revolution speeds of a first and a second main shafts,which change takes place at the time of up-shifting while the vehicleruns in second gear. FIG. 10 is a diagram showing a power-transmissionroute at the time when synchronizing means operates at the up-shiftingwhile the vehicle runs in second gear.

FIG. 1 shows a power unit P to be mounted, for example, on a motorcycle,and includes an engine E and a transmission system M. The engine E is,for example, a four-cylinder engine, while the transmission system Mchanges the speed of the output of the engine E and transmits the outputto the rear wheel, which is not illustrated here. The transmissionsystem M includes a gear transmission 7, which has a plurality of geartrains, such as a first- to a fifth-speed gear trains G1 to G5, and withwhich a speed can be selected to be established from a plurality ofspeeds, and also includes a clutch system 8 provided between the engineE and the gear transmission 7.

Now, refer also to FIG. 2. The torque of a crankshaft 9 that the engineE has (see FIG. 1) is received by the clutch system 8 via a primaryreduction system 11 and a damper spring 12. The clutch system 8 switchesfrom allowing to blocking, or vice versa, the transmission of the torquefrom the crankshaft 9, via the primary reduction system 11 and thedamper spring 12, to the gear transmission 7.

A crankcase 14 is provided to an engine body 10. In the crankcase 14, aright-hand sidewall 14 a and a left-hand sidewall 14 b are locatedrespectively on the right and the left sides with respect to thedirection in which the motorcycle advances. Meanwhile a middle wall 14 cis located in a middle portion between the right-hand and the left-handsidewalls 14 a and 14 b. In the crankcase 14, a transmission housing 13is formed between the left-hand sidewall 14 b and the middle wall 14 cto accommodate the gear transmission 7.

The gear transmission 7 has a first main shaft 15, a second main shaft16, and a countershaft 17, all of which are rotatably supported by thecrankcase 14. The cylindrical first main shaft 15 has an axis parallelto the crankshaft 9. The second main shaft 16 coaxially penetrates thefirst main shaft 15. The second main shaft 16 can rotate relatively tothe first main shaft 15 while the relative positions of the two shafts15 and 16 are fixed in the axial direction.

The second main shaft 16 penetrates the right-hand sidewall 14 a, themiddle wall 14 c, and the left-hand sidewall 14 b with a first endportion of the second main shaft 16 sticking out from the right-handsidewall 14 a of the crankcase 14 and a second end portion sticking outfrom the left-hand sidewall 14 b. Thus, the second main shaft rotatesfreely. A transmission cylinder shaft 18, which is coaxially joined withthe first main shaft 15 while being not allowed to rotate relatively tothe first main shaft 15, coaxially surrounds the second main shaft 16.The transmission cylinder shaft 18 penetrates a support ring 26 fastenedto the right-hand sidewall 14 a of the crankcase 14, and rotates freely.A ball bearing 19 is set between the transmission cylinder shaft 18 andthe support ring 26. In addition, needle bearings 20 . . . are setbetween the transmission cylinder shaft 18 and the second main shaft 16while a ball bearing 21 is set between the left-hand sidewall 14 b andthe second main shaft 16. The first main shaft 15 penetrates the middlewall 14 c of the crankcase 14, and rotates freely. A ball bearing 22 isset between the middle wall 14 c and the first main shaft 15 whileneedle bearings 23 . . . are set between the first and the second mainshafts 15 and 16.

A first end portion of the countershaft 17 is rotatably supported by themiddle wall 14 c of the crankcase 14 with a ball bearing 24 interposedin between. Meanwhile, a second end portion of the countershaft 17penetrates the left-hand sidewall 14 b of the crankcase 14, and rotatesfreely. A ball bearing 25 is set between the left-hand sidewall 14 b andthe countershaft 17.

Now, refer also to FIG. 3. The clutch system 8 includes a first and asecond hydraulic clutches 28 and 29 of the multi-plate type. The firsthydraulic clutch 28 switches from allowing to blocking, or vice versa,the transmission of the driving power from the primary reduction system11 to the first main shaft 15. Meanwhile the second hydraulic clutch 29switches from allowing to blocking, or vice versa, the transmission ofthe driving power from the primary reduction system 11 to the secondmain shaft 16.

The primary reduction system 11 is composed of a drive gear 30 on thecrankshaft 9 and a driven gear 31 meshing with the drive gear 30. Acylindrical first boss 32 coaxially surrounds and is joined to the firstmain shaft 15 while not allowed to rotate relatively to the first mainshaft 15. The driven gear 31 is supported by the first boss 32 with aroller bearing 33 interposed in between, and thus rotates freely.

An input member 34 is provided to and shared by the first and the secondclutches 28 and 29. The input member 34 includes an annular plateportion 34 a, an inner cylindrical portion 34 b, and an outercylindrical portion 34 c. The annular plate portion 34 a is locatedadjacently to the primary reduction system 11 at its outer side in theaxial direction of the second main shaft 16. On the inner perimeter sideof the annular plate portion 34 a and at the side opposite the sidewhere the primary reduction system 11 is located, the inner cylindricalportion 34 b is, integrally and continuously, formed with the base endof the inner cylindrical portion 34 b being perpendicular to the annularplate portion 34 a. The outer cylindrical portion 34 c is, integrallyand continuously, formed on the outer perimeter side of the annularplate portion 34 a with the base end of the outer cylindrical portion 34c being perpendicular to the annular plate portion 34 a. The outercylindrical portion 34 c, thus formed, coaxially surrounds the innercylindrical portion 34 b. In the annular plate portion 34 a of the inputmember 34, joint bosses 34 d are integrally formed extending from aplurality of positions along the circumferential direction. The jointbosses 34 d are located at the opposite side of the annular plateportion 34 a from the inner cylindrical portion 34 b and the outercylindrical portion 34 c. Long holes 35, each of which has a shapeelongated in the circumferential direction, are formed in the drivengear 31. The joint bosses 34 d are respectively inserted into the longholes 35. A retention plate 36, which faces the driven gear 31 at theopposite side thereof from the input member 34, abuts on the jointbosses 34 d. The retention plate 36 is joined to the end faces of thejoint bosses 34 d with respective rivets 37, each of which penetratesthe corresponding one of the joint bosses 34 d. In addition, a discspring 38 is provided between the retention plate 36 and the driven gear31 for generating a spring force to make the driven gear 31 abut on theannular plate portion 34 a of the input member 34.

In the driven gear 31, retention holes 39, each of which has a shapeelongated in the circumferential direction, are formed in a plurality ofpositions, which are shifted, in the circumferential direction, from thepositions of the long holes 35. The damper spring 12 is accommodated andheld in each of the retention holes 39 so as to be enclosed by thedriven gear 31, the input member 34, and the retention plate 36.

The first hydraulic clutch 28 includes the above-described innercylindrical portion 34 b of the input member 34, a first output member40, a plurality of first clutch discs 41, a plurality of first clutchplates 42, a first pressure-receiving plate 43, a first clutch piston44, and a first clutch spring 45. The first output member 40 has a firstcylindrical portion 40 a, which coaxially surrounds the innercylindrical portion 34 b. The first output member 40 is fixed to thefirst boss 32, and is thus joined to the first main shaft 15 so as notto rotate relatively to the first main shaft 15. The first clutch discs41 engage with the first cylindrical portion 40 a so as not to rotaterelatively to the first cylindrical portion 40 a. The first clutchplates 42 engage with the inner cylindrical portion 34 b so as not torotate relatively to the inner cylindrical portion 34 b, and arearranged as being alternately stacked with the first clutch discs 41.The first pressure-receiving plate 43 faces the first clutch discs 41and the first clutch plates 42, which are arranged as being alternatelystacked with each other, from the side where the annular plate portion34 a is located. The first clutch piston 44 has an annular firstpressure plate portion 44 a, and is located on the opening end side ofthe input member 34. The first pressure plate portion 44 a faces thefirst clutch discs 41 and the first clutch plates 42, which are arrangedas being alternately stacked with each other, from the side opposite theside where the annular plate portion 34 a is located. The first clutchspring 45 generates a spring force that biases the first pressure plateportion 44 a so as to move away from the first pressure-receiving plate43.

The first clutch piston 44 has the annular first pressure plate portion44 a at the outer circumference thereof while the inner circumferentialportion of the first clutch piston 44 is, slidingly and liquid-tightly,in contact with the outer circumference of the first boss 32. The firstoutput member 40 has a joint plate portion 40 b, which is a dish-likeportion facing the first clutch piston 44 from the outer side. While theinner circumferential portion of the joint plate portion 40 b is weldedto the first boss 32, an end of the first cylindrical portion 40 a islinked to and integrated with the outer circumference of the joint plateportion 40 b. The outer circumference of the first pressure plateportion 44 a of the first clutch piston 44 is, slidably andliquid-tightly, fitted into the inner surface of the outer circumferenceportion of the joint plate portion 40 b. A first hydraulic chamber 46 isthus formed between the first clutch piston 44 and the first outputmember 40. The first clutch spring 45 is provided between the firstclutch piston 44 and a first retainer 47, which is fitted onto the outercircumference of the first boss 32. The first clutch spring 45 thusprovided biases the first clutch piston 44 so as to reduce the volume ofthe first hydraulic chamber 46. In addition, a stopper ring 48 is fittedto the inner surface of the first cylindrical portion 40 a near the openend thereof, and abuts on the outer periphery of the firstpressure-receiving plate 43 from the opposite side of the plate 43 fromthe first pressure plate portion 44 a.

In the first hydraulic clutch 28 with this structure, as the oilpressure in the first hydraulic chamber 46 increases, the first clutchdiscs 41 and the first clutch plates 42, which are alternately stacked,are pressed by and between the first pressure plate portion 44 a and thefirst pressure-receiving plate 43, and thus frictionally engage witheach other. Accordingly, the driving power is transmitted from the innercylindrical portion 34 b of the input member 34 to the first outputmember 40 while the driving power of the crankshaft 9 is transmittedfrom the first boss 32, via the transmission cylinder shaft 18, to thefirst main shaft 15.

The second hydraulic clutch 29 includes the outer cylindrical portion 34c of the input member 34, a second output member 50, a plurality ofsecond clutch discs 51, a plurality of second clutch plates 52, anannular second pressure-receiving plate 53, a second clutch piston 54,and a second clutch spring 55. The second output member 50 has a secondcylindrical portion 50 a, which is provided inside the outer cylindricalportion 34 c, and which coaxially surrounds the first cylindricalportion 40 a of the first hydraulic clutch 28, and is thus joined to thesecond main shaft 16 so as not to rotate relatively to the second mainshaft 16. The second clutch discs 51 engage with the outer cylindricalportion 34 c so as not to rotate relatively to the outer cylindricalportion 34 c. The second clutch plates 52 engage with the secondcylindrical portion 50 a so as not to rotate relatively to the secondcylindrical portion 50 a, and are arranged as being alternately stackedwith the second clutch discs 51. The second pressure-receiving plate 53faces the second clutch discs 51 and the second clutch plates 52, whichare arranged as being alternately stacked with each other, from the sidewhere the annular plate portion 34 a is located. The second clutchpiston 54 has an annular second pressure plate portion 54 a, which facesthe second clutch discs 51 and the second clutch plates 52, which arearranged as being alternately stacked with each other, from the sideopposite the side where the annular plate portion 34 a is located. Thesecond clutch spring 55 generates a spring force that biases the secondpressure plate portion 54 a so as to move away from the secondpressure-receiving plate 53.

A cylindrical second boss 59, which coaxially surrounds the second mainshaft 16, is joined to an end portion of the second main shaft 16 as notbeing allowed to rotate relatively to the second main shaft 16. Thesecond output member 50 has a joint plate portion 50 b, which is adish-like portion facing the joint plate portion 40 b of the firstoutput member 40 in the first hydraulic clutch 28 from the outer side.While the inner circumferential portion of the joint plate portion 50 bis welded to the second boss 59, an end of the second cylindricalportion 50 a is linked to and integrated with the outer circumference ofthe joint plate portion 50 b. The second clutch piston 54 has theannular second pressure plate portion 54 a formed on the outercircumference thereof, and faces the joint plate portion 50 b of thesecond output member 50 from outside. The inner circumferential portionof the second clutch piston 54 is, slidingly and liquid-tightly, incontact with the outer circumference of the second boss 59. The firstclutch piston 44 of the first clutch 28 and the second clutch piston 54,which has a larger diameter than the first clutch piston 44, arearranged side by side in the axial direction.

In addition, the outer circumference of the second pressure plateportion 54 a of the second clutch piston 54 is, slidably andliquid-tightly, fitted into a third cylindrical portion 60 a, formed onthe outer perimeter of a dish-shaped case member 60. The innercircumference of the case member 60 is liquid-tightly fixed to thesecond boss 59, and thus a second hydraulic chamber 56 is formed betweenthe second clutch piston 54 and the case member 60. The second clutchspring 55 is provided between the second clutch piston 54 and a secondretainer 57, which is fitted into the joint plate portion 50 b of thesecond output member 50. The second clutch spring 55 thus providedbiases the second clutch piston 54 so as to reduce the volume of thesecond hydraulic chamber 56. In addition, a stopper ring 58 is fittedonto the outer surface of the second cylindrical portion 50 a near theopen end thereof, and abuts on the outer periphery of the secondpressure-receiving plate 53 from the opposite side of the plate 53 fromthe second pressure plate portion 54 a.

In the second hydraulic clutch 29 with this structure, as the oilpressure in the second hydraulic chamber 56 increases, the second clutchdiscs 51 and the second clutch plates 52, which are alternately stacked,are pressed by and between the second pressure plate portion 54 a andthe second pressure-receiving plate 53, and thus frictionally engagewith each other. Accordingly, the power is transmitted from the outercylindrical portion 34 c of the input member 34 to the second outputmember 50 while the power of the crankshaft 9 is transmitted from thesecond boss 59 to the second main shaft 16.

Incidentally, first ends of the first and the second main shafts 15 and16 are connected respectively to the first and the second hydraulicclutches 28 and 29 of the clutch system 8 outside of the right-handsidewall 14 a of the crankcase 14. A clutch cover 61, which covers theclutch system 8, is fastened to the right-hand sidewall 14 a.

As shown in FIG. 4, an engine cover 63 is provided to cover theleft-hand sidewall 14 b of the crankcase 14. In the engine cover 63, apassage hole 65 is coaxially formed with the first and the second mainshafts 15 and 16. The passage hole 65 has an end-wall 64 at the innerend thereof while a cap 66 is provided to liquid-tightly close the outerend of the passage hole 65.

A wall member 67 is liquid-tightly fitted into the passage hole 65. Afirst operating-fluid supply chamber 68 is thus formed between the wallmember 67 and the end wall 64 while a second operating-fluid supplychamber 69 is formed between the wall member 67 and the cap 66. Inaddition, a center hole 70 with a base, the center hole 70 having afirst end closed and a second end open, is coaxially formed in thesecond main shaft 16. An inner supply pipe 71, which is coaxiallyinserted into the center hole 70, has a first end portion placed in thevicinity of the closed-end of the central hole 70. A second end portionof the inner supply pipe 71 penetrates the end wall 64, and isliquid-tightly held by the wall member 67. An annular seal member 72(see FIG. 3) is set between the first end portion of the inner supplypipe 71 and the inner circumferential surface of the center hole 70.Furthermore, an outer supply pipe 73 is inserted into the central hole70 while coaxially surrounding the inner supply pipe 71, and extendsuntil reaching the vicinity of the first end portion of the inner supplypipe 71. An annular seal member 74 (see FIG. 3) is set between a firstend portion of the outer supply pipe 73 and the inner circumferentialsurface of the center hole 70. Meanwhile, a second end portion of theouter supply pipe 73 liquid-tightly penetrates the left-hand sidewall 14b of the crankcase 14, and then is liquid-tightly fitted into and heldby the end wall 64 of the passage hole 65.

Now, attention should be focused on FIG. 3. A plurality of firstcommunication passages 75, which lead to the first hydraulic chamber 46of the first hydraulic clutch 28, are radially formed so as to penetratethe first boss 32, the transmission cylinder shaft 18, and the secondmain shaft 16. An annular first hydraulic passage 76, which allows thesefirst communication passages 75 to communicate with the firstoperating-fluid supply chamber 68, is formed between the inner supplypipe 71 and the outer supply pipe 73. In addition, a plurality of secondcommunication passages 77, which lead to the second hydraulic chamber 56of the second hydraulic clutch 29, are radially formed so as topenetrate the second boss 59. A second hydraulic passage 78, whichallows these second communication passages 77 to communicate with thesecond operating-fluid supply chamber 69, is formed in the inner supplypipe 71 and together with an inner end portion of the center hole 70.

In this way, inside the second main shaft 16, the first and the secondhydraulic passages 76 and 78 are concentrically formed, and enablehydraulic pressure to be individually applied to the first and thesecond clutch pistons 44 and 54 respectively of the first and the secondhydraulic clutches 28 and 29 in the clutch system 8. In this case, theoperating fluid is supplied from the second-end side of the second mainshaft 16.

A hydraulic pressure control means 100, which controls the supply of theoperating fluid to the first and the second hydraulic passage 76 and 78,is attached to the opposite side surface of the engine E from the clutchsystem 8, that is, to the outer surface of the engine cover 63.

Now, refer also to FIG. 5. The hydraulic pressure control means 100includes a first solenoid valve 101, which controls the supply ofoperating fluid to the first hydraulic passage 76, that is, to the firstoperating-fluid supply chamber 68. The hydraulic pressure control means100 also includes a second solenoid valve 102, which controls the supplyof operating fluid to the second hydraulic passage 78, that is, to thesecond operating-fluid supply chamber 69. The first and the secondsolenoid valves 101 and 102 share a common valve body 103, which isfastened to a support plate 105 by a plurality of bolts 104. To put itanother way, a common support plate 105 supports both the first and thesecond solenoid valves 101 and 102, while the support plate 105 isjoined to the outer surface of the engine cover 63 by a plurality ofbolts 106, 106 in a position above the cap 66.

A first communicating-fluid passage 107 links the first solenoid valve101 of the hydraulic pressure control means 100 with the firstoperating-fluid supply chamber 68, while a second communicating-fluidpassage 108 links the second solenoid valve 102 with the secondoperating-fluid supply chamber 69. At least one of the first and thesecond communicating-fluid passages 107 and 108—in this embodiment thefirst communicating-fluid passage 107—has a part formed between thejoint surfaces of the engine cover 63 and the support plate 105. Forthis purpose, a groove 109 is formed in the outer surface of the enginecover 63 to form a part of the first communicating-fluid passage 107between the engine cover 63 and the support plate. A connecting-pipepassage 110, which forms a part of the second communicating-fluidpassage 108, has a first end joined to the support plate 105 andconnected to the second solenoid valve 102. Meanwhile, a second end ofthe connecting-pipe passage 110 is joined to the engine cover 63 in aposition near the second operating-fluid supply chamber 69.

An oil filter 112 is attached to the engine cover 63 below the hydraulicpressure control means 100. A shift-control motor 113 is attached to theengine cover 63 near and below the cap 66, and generates the power toswitch the state of establishments of the first to the fifth gear trainsG1 to G5 in the gear transmission 7. The power transmitted through thegear transmission 7 drives to rotate an output shaft 111, which issupported by and sticks out of the engine cover 63 in a position next tothe hydraulic pressure control means 100.

Now, attention should be focused on FIG. 4. In the gear transmission 7,each of the second and the fourth gear trains G2 and G4 is establishedby a gear on the first main shaft 15 and one on the counter shaft 17while each of the first, the third, and the fifth gear trains G1, G3,and G5 is established by a gear on the second main shaft 16 and thecounter shaft 17. A damper spring 81 and a secondary reduction system 82transmit, to the output shaft 111, the power outputted from the secondend portion of the countershaft 17, which end portion sticks out of theleft-hand sidewall 14 b of the crankcase 14.

The second-speed gear train G2 includes a second-speed drive gear 83,which is integrally provided on the first main shaft 15, and asecond-speed driven idle gear 84, which is supported on the countershaft17 so as to rotate relatively to the countershaft 17, and which mesheswith the second-speed drive gear 83. The fourth-speed gear train G4includes a fourth-speed drive gear 85, which is fixed on the first mainshaft 15, and a fourth-speed driven idle gear 86, which is supported onthe countershaft 17 so as to rotate relatively to the countershaft 17,and which meshes with the fourth-speed drive gear 85.

A first shifter 87 is splined to fit onto the countershaft 17 betweenthe second- and the fourth-speed driven idle gears 84 and 86. The statethat can be selected for the first shifter 87 is one of the following:the first shifter 87 engages with any one of the second- and thefourth-speed driven idle gears 84 and 86, and the first shifter 87engages neither one of the second- and the fourth-speed driven idlegears 84 and 86. The movement of the first shifter 87 in the axialdirection enables the switching between a neutral state and a statewhere any one of the second and the fourth gear trains are established.In the neutral state, both of the second- and the fourth-speed drivenidle gears freely rotate relatively to the countershaft 17. In the statein which any one of the second- and the fourth-speed gear trains areestablished, the relevant one of the second- and the fourth-speed drivenidle gears 84 and 86 is joined to the countershaft 17 so as not torotate relatively to the countershaft 17.

The first-speed gear train G1 includes a first-speed drive gear 88,which is integrally provided on the second main shaft 16, and afirst-speed driven idle gear 89, which is supported on the countershaft17 so as to freely rotate relatively to the countershaft 17, and whichmeshes with the first-speed drive gear 88. The third-speed gear train G3includes a third-speed drive gear 90, which is splined to fit onto thesecond main shaft 16. The third-speed drive gear 90 is allowed to slidein the axial direction of the second main shaft 16, but is not allowedto rotate relatively to the second main shaft 16. The third-speed geartrain G3 also includes a third-speed driven idle gear 91, which issupported on, so as to freely rotate relatively to, the countershaft 17and which meshes with the third-speed drive gear 90. The fifth-speedgear train G5 includes a fifth-speed drive idle gear 92, which issupported on, so as to freely rotate relatively to, the second mainshaft 16. The fifth-speed gear train G5 also includes a fifth-speeddriven gear 93, which is supported on, so as not to rotate relativelyto, the countershaft 17, and which meshes with the fifth-speed driveidle gear 92.

The third-speed drive gear 90 is integrally formed with a second shifter94. The second shifter 94 is splined to fit onto the second main shaft16 while the second shifter 94 enables the switching between a statewhere the second shifter 94 engages with the fifth-speed drive idle gear92 and a state the two are disengaged. The fifth-speed driven gear 93 isintegrally formed with a third shifter 95. The third shifter 95 issplined to fit onto the countershaft 17 while the third shifter 95enables the switching between a state where the third shifter 95 engageswith any one of the first- and the third-speed driven idle gears 89 and91 supported on, as allowed to rotate relatively to, the countershaft17, and a state where the third shifter engages with none of the first-and the third-speed driven idle gears 89 and 91.

Moving the second and the third shifters 94 and 95 in the axialdirection enables the switching among the following states: a neutralstate, where the fifth-speed drive idle gear 92 freely rotatesrelatively to the second main shaft 16 while the first- and thethird-speed driven idle gears 89 and 91 freely rotate relatively to thecountershaft 17; a state where the first-speed driven idle gear 89 isjoined to, so as not to rotate relatively to, the countershaft 17, sothat the first-speed gear train G1 is established; a state where thethird-speed drive gear 90 is joined to, so as not to rotate relativelyto, the second main shaft 16 while the third-speed driven idle gear 91is joined to, so as not to rotate relatively to, the countershaft 17, sothat the third-speed gear train G3 is established; and a state where thefifth-speed drive idle gear 92 is joined to, as not allowed to rotaterelative to, the second main shaft 16, so that the fifth-speed geartrain G5 is established.

The first, the second, and the third shifters 87, 94, and 95 arerotatably held respectively by a first, a second, and a third shiftforks 96, 97, and 98. When the shift-control motor 113 is activated tomove rotationally a shift drum (not illustrated), the first to the thirdshift forks 96 to 98 as well as the shifters 87, 94, and 95 areslidingly operated in the axial direction to selectively establish thefirst- to the fifth-speed gear trains G1 to G5.

Incidentally, engagement and disengagement of the first and the secondhydraulic clutch 28 and 29 of the clutch system 8 are switchedalternatively. When the gear-speed is changed from one to another withthe power from the engine E being transmitted to any one of the firstand the second main shafts 15 and 16 of the gear transmission 7, for thepurpose of a preliminary gear shift, one of the gear trains between thecountershaft 17 and the other one of the first and the second mainshafts 15 and 16 is established. For example, suppose that a vehicleruns in second speed, that is, the second-speed gear train G2 isestablished in a state where the power from engine E is transmitted tothe first main shaft 15 with the first hydraulic clutch 28 beingengaged. In this case, before down-shifting to first speed is carriedout, the third shifter 93 is slidingly operated towards the side wherethe third shifter 93 can engage with the first-speed driven idle gear89. Thus established is the first-speed gear train G1 between thecountershaft 17 and the second main shaft 16 linked to the secondhydraulic clutch 29, in which the power transmission has been cut offthus far. Meanwhile, before up-shifting to the third gear, the thirdshifter 93 is slidingly moved towards the side where the third shiftercan engage with the third-speed driven idle gear 91. Thus established isa third-speed gear train G3 between the countershaft 17 and the secondmain shaft 16.

Each of these idle gears constitutes a part of one of the gear trains tobe established for the preliminary transmission at the time of changingthe speed gear, and is rotatably supported on the second main shaft 16or on the counter shaft 17. Each of the shifters is supported on any oneof the second main shaft 16 and the countershaft 17, while the shiftersare allowed to slide on the corresponding shafts, but the shifters arenot allowed to rotate relatively to the corresponding shafts. A largedifference of revolutions between one of the idle gears and thecorresponding one of the shifters produces a large noise when the idlegear and the shifter engage with each other. In response to thisproblem, single synchronizing means 116 is provided to give accelerationtorque or deceleration torque to one of the first and the second mainshafts 15 and 16, precisely, to the one to which the power transmissionfrom the engine E has been cut off.

The synchronizing means 116 includes an interlocking rotation shaft 117,a first gear-transmission mechanism 118, a second gear-transmissionmechanism 119, and a switching mechanism 120. The interlocking rotationshaft 117 rotates in conjunction with one of the first and the secondmain shafts 15 and 16—in this embodiment, with the second main shaft 16.The first and the second gear-transmission mechanisms 118 and 119 arelinked to the other one of the first and the second main shafts 15 and16—in this embodiment, the first main shaft 15. While the firstgear-transmission mechanism 118 increases the speed of the torquetransmitted from the first main shaft 15, the second gear-transmissionmechanism 119 decreases the speed of the torque. The switching mechanism120 switches among the alternative linking of the interlockingrotational shaft 117 with either one of the first and the secondgear-transmission mechanisms 118 and 119, and the de-linking therewith.

In each configuration of FIGS. 2 to 6, the interlocking rotation shaft117 appears above the first and the second main shafts 15 and 16 for theconvenience of the drawing, but is actually placed below the two mainshafts 15 and 16. In addition, the interlocking rotation shaft 117 hasan axis parallel to the first and the second main shafts 15 and 16. Afirst end of the interlocking rotation shaft 117 is rotatably supportedby a support member 125 with a ball bearing 121 interposed in betweenwhile a second end of the interlocking rotation shaft 117 is rotatablysupported by the left-hand sidewall 14 b of the crankcase 14 with a ballbearing 122 interposed in between. Here, the support member 125 isfastened to the support ring 26, which is fastened to the right-handsidewall 14 a, and which supports the transmission cylinder shaft 18.

The first and the second gear-transmission mechanisms 118 and 119 areconnected to the first main shaft 15, and are placed between the first-to the fifth-speed gear trains G1 to G5 and the clutch system 8, whichincludes the first and the second hydraulic clutches 28 and 29. Thefirst gear-transmission mechanism 118 includes a first large-diametergear 123 and a first small-diameter gear 124, which meshes with thefirst large-diameter gear 123. The first large-diameter gear 123 isintegrally provided in a position near an end of the transmissioncylinder shaft 18, which end is the opposite one from the location ofthe clutch system 8. The transmission cylinder shaft 18 is connected to,so as not to rotate relatively to, the first main shaft 15. Onto theopposite end portion of the transmission cylinder shaft 18 from theclutch system 8, a second small-diameter gear 125 is integrallyprovided. The second small-diameter gear 125 and a second large-diametergear 126, which meshes with the second small-diameter gear 125,constitute the second gear-transmission mechanism 119.

Note that the first and the second large-diameter gears 123 and 126 havethe same diameter while the first and the second small-diameter gears124 and 125 have the same diameter. The first and the secondlarge-diameter gears 123 and 126 have a larger diameter than the firstand the second small-diameter gears 124 and 125 have.

At the first end side of the interlocking rotation shaft 117, alarge-diameter portion 117 a is formed coaxially and integrallytherewith. The large-diameter portion 117 a supports the firstsmall-diameter gear 124 and the second large-diameter gear 126, and doesnot allow the rotation of these gears 124 and 126 relative to thelarge-diameter portion 117 a. The switching mechanism 120 switches amonga first state where the first small-diameter gear 124 is joined to, soas not to rotate relatively to, the large-diameter portion 117 a, asecond state where the large-diameter gear 126 is joined to, so as notto rotate relatively to, the large-diameter portion 117 a, and a thirdstate where neither the first small-diameter gear 124 nor the secondlarge-diameter gear 126 is joined to the large-diameter portion 117 a.

A conventionally known synchromesh mechanism is employed for theswitching mechanism 120, and no detail description will be given here.The switching among the first to the third states is carried out bysliding a sleeve 127 in the axial direction of the large-diameterportion 117 a.

The interlocking rotation shaft 117 is interlocked and joined with thesecond end portion of the second main shaft 16 in a position in the geartransmission 8 so that the first to the fifth gear trains G1 to G5 areset with the joining position at one side, and the first and the secondgear-transmission mechanism 118 and 119 at the other side. In otherwords, a gear 128 provided on the second end portion of the interlockingrotation shaft 117 meshes with a gear 129 fixed on the second main shaft16 between the left-hand sidewall 14 b of the crankcase 14 and thefirst-speed drive gear 88. The two gears are formed to have the samediameter.

Now, refer also to FIG. 6. An actuator 130, which drives the sleeve 127of the switching mechanism 120, is placed at a side of the clutch system8, which has the first and the second hydraulic clutches 128 and 129.The actuator has a hydraulic cylinder 131 and a pair of electromagneticswitching valves 132 and 133, which control the operation of thehydraulic cylinder 131.

A cylinder body 134 and a piston 135 slidably fitted thereintoconstitute the hydraulic cylinder 131, which is placed below theswitching mechanism 120 and between the switching mechanism 120 and theclutch system 8. Inside the cylinder body 134, a first and a secondcontrol hydraulic chambers 137 and 138 are formed to face the two endsof the piston 135 respectively. Springs 139 and 140 are accommodated inthe first and the second control hydraulic chambers 137 and 138respectively to bias the piston 135 towards the neutral position.

An increased hydraulic pressure of the first control hydraulic chamber137 and a decreased hydraulic pressure of the second control hydraulicchamber 138 move the piston 135 to a side, and result in a decrease inthe volume of the second control hydraulic chamber 138. Thus, a pistonrod 141 and a fork 142 operate so as to move the sleeve 127 to a side,where the sleeve 127 connects the first gear-transmission mechanism 118with the interlocking rotation shaft 117 while not allowing theirrelative rotation to each other. In contrast, an increased hydraulicpressure of the second control hydraulic chamber 138 and a decreasedhydraulic pressure of the first control hydraulic chamber 137 move thepiston 135 to another side, and result in a decrease in the volume ofthe first control hydraulic chamber 137. Thus, the piston rod 141 andthe fork 142 operate so as to move the sleeve 127 to another side, wherethe sleeve 127 links the second gear-transmission mechanism 119 with theinterlocking rotation shaft 117 while not allowing their relativerotation to each other.

The first and the second electromagnetic switching valves 132 and 133are placed below the clutch system 8, and are attached to an oil pan145. It should be noted that the oil pan 145 appears above the crankcase14 for the convenience of the drawing, but is actually joined to thebottom of the crankcase 14 as FIG. 6 shows.

A fluid passage 148 is provided in the oil pan 145 so as to beconnected, via a connection pipe 147, to an operating-fluid supplypassage 146 provided in the crankcase 14. A supply passage 149, which isformed between the oil pan 145 and a lid plate 151 fastened thereto,leads to the first control hydraulic chamber 137. A relief passage 150is provided in the oil pan 145, and has an opening inside the crankcase14. The first electromagnetic switching valve 132 controls—increases anddecreases—the hydraulic pressure of the first control hydraulic chamber137 so as to alternatively switch between a state where the fluidpassage 148 communicates to the supply passage 149 and another statewhere the relief passage 150 communicates to the supply passage 149.

The second electromagnetic valve 133 controls—increases anddecreases—the hydraulic pressure of the second control hydraulic chamber138 so as to alternatively switch a state where the fluid passage 148 inthe oil pan 145 communicates to the second control hydraulic chamber 138and another state where the hydraulic pressure of the second controlhydraulic chamber 138 is relieved to the inside of the crankcase 14.

Next, what follows is a description of the operation of this embodiment.The first- to the fifth-speed gear trains G1 to G5, each of which can beselectively established, are provided between the countershaft 17, andthe first and the second main shafts 15 and 16. Alternative switching ispossible so that the power from the engine E can be transmitted eitherone of the first and second main shafts 15 and 16. Assuming that thepower from the engine E is transmitted any one of the first and thesecond main shaft 15 and 16, and that a gear shifting is carried out. Atthis time, as a preliminary gear shifting, one of the gear trains isestablished between the countershaft 17 and the other one of the mainshafts 15 and 16, that is, the one that the power from the engine E isnot transmitted to. Meanwhile, the single synchronizing means 116, whichserves commonly to the gear trains G1 to G5, gives decelerating oraccelerating torque to this one of the main shafts 15 and 16, the onethat the power from the engine E is not transmitted to.

The above-described operation produces a smaller difference inrevolution rate between the members that engage with each other when oneof the gear trains is established. This, in turn, results in a smallernoise produced at the time of engagement. Assuming that the vehicle runsat a constant speed in second gear, when the power from the engine E istransmitted to the first main shaft 15 with the first hydraulic clutch28 connected. The first-speed gear train G1 has to be establishedbetween the second main shaft 16 and the countershaft 17 before thedown-shifting to the first gear is carried out. To this end, the thirdshifter 93 operates and slides to a side so as to engage with thefirst-speed driven idle gear 89. In this case, the synchronizing means116 gives, to the second main shaft 16, to which the power from theengine E is not transmitted, torque that comes from the first main shaft15 but in a revolution rate increased by the first gear-transmissionmechanism 118.

While the vehicle runs at a second speed, the second main shaft 16,dragged by the second hydraulic clutch 29 with no power from engine Etransmitted thereto, revolves at a revolution rate NB, which isapproximately equal to a revolution rate NA of the first main shaft 15,as FIG. 7 shows. In this case, the establishment of the first-speed geartrain G1 at a time t3, makes the revolution rate NB of the second mainshaft 16 drastically increase from the revolution rate NA of the firstmain shaft 15 as the dot-dash line in FIG. 7 shows unless thesynchronizing means 116 gives torque to the second main shaft 16. Thisdrastic increase derives from the difference in the speed reducing ratiobetween the second- and the first-speed gear trains G2 and G1. While thethird shifter 93 revolves together with the countershaft 17, thefirst-speed driven idle gear 89 is made to revolve about the axis of thecountershaft 17 by the power transmitted from the second main shaft 16.The first-speed driven idle gear 89 revolves at a lower rate than thethird shifter 93 does before the engagement of these two, and thedifference in the revolution rate produces a big engagement noise.

Assuming that the actuator 130 of the synchronizing means 116 starts tooperate at the time t1 prior to the time t3, and that the linkingoperation of the first gear-transmission mechanism 118 with theinterlocking rotation shaft 117 is completed at the time t2 prior to thetime t3. Then, as one of the arrows in FIG. 8 shows, the torque with aspeed increased by the first gear-transmission mechanism 118 of thesynchronizing means 116 is transmitted to the second main shaft 16, andthus the revolution rate of the second main shaft 16 begins to increase.As a result, the revolution rate of the second main shaft 16 increases,by the time t2, up to a rate close to the rate at which the second mainshaft 16 eventually revolves when the first gear train G1 is establishedat the time t3.

As described above, in comparison with the amount of change a in therevolution rate of the second main shaft 16 at the time t3 in a casewhere no torque with its speed increased is given thereto by thesynchronizing means 116, the amount of change b in the revolution rateof the second main shaft 16 at the time t3 in a case where torque withits speed increased is given thereto by the synchronizing means 116 issignificantly small. Accordingly, when, in a vehicle running at aconstant speed in second gear, a preliminary gear shifting is carriedout to establish the first-speed gear train G1 in preparation for thedown-shifting, the difference in rotation rate between the third shifter93 and the first-speed driven idle gear 89 is made small, and thus thenoise, generated when the third shifter 93 and the first-speed drivenidle gear 89 are engaged, is also made small.

Now, assuming that the vehicle runs at a constant speed in second gear,when the power from the engine E is transmitted to the first main shaft15 with the first hydraulic clutch 28 connected. The third-speed geartrain G3 has to be established between the second main shaft 16 and thecountershaft 17 before the up-shifting to the third gear is carried out.To this end, the third shifter 93 operates and slides to a side so as toengage with the third-speed driven idle gear 91. In this case, theestablishment of the third-speed gear train G3 at a time t3 makes therevolution rate NB of the second main shaft 16 drastically decrease fromthe revolution rate NA of the first main shaft 15 as the dot-dash linein FIG. 9 shows unless the synchronizing means 116 gives torque to thesecond main shaft 16. This drastic decrease derives from the differencein the speed reducing ratio between the second- and the first-speed geartrains G2 and G3. While the third shifter 93 revolves together with thecountershaft 17, the third-speed driven idle gear 91 is made to revolveabout the axis of the countershaft 17 by the power transmitted from thesecond main shaft 16. The third-speed driven idle gear 91 revolves at ahigher rate than the third shifter 93 does before the engagement ofthese two, and the difference in the revolution rate produces a bigengagement noise.

Assuming that the actuator 130 of the synchronizing means 116 starts tooperate at the time t1 prior to the time t3, and that the linkingoperation of the second gear-transmission mechanism 119 with theinterlocking rotation shaft 117 is completed at the time t2 prior to thetime t3. Then, as one of the arrows in FIG. 8 shows, the torque with aspeed decreased by the second gear-transmission mechanism 119 of thesynchronizing means 116 is transmitted to the second main shaft 16, andthus the revolution rate of the second main shaft 16 begins to decrease.As a result, the revolution rate of the second main shaft 16 decreases,by the time t2, down to a rate close to the rate at which the secondmain shaft 16 eventually revolves when the third gear train G3 isestablished at the time t3.

As described above, in comparison with the amount of change c in therevolution rate of the second main shaft 16 at the time t3 in a casewhere no torque with its speed decreased is given thereto by thesynchronizing means 116, the amount of change d in the revolution rateof the second main shaft 16 at the time t3 in a case where torque withits speed decreased is given thereto by the synchronizing means 116 issignificantly small. Accordingly, when, in a vehicle running at aconstant speed in second gear, a preliminary gear shifting is carriedout to establish the third-speed gear train G3 in preparation for theup-shifting, the difference in rotation rate between the third shifter93 and the third-speed driven idle gear 91 is made small, and thus thenoise, generated when the third shifter 93 and the third-speed drivenidle gear 91 are engaged, is also made small.

In addition, the synchronizing means 116 is a single unit commonlyserves for all the gear trains G1 to G5, which results in a smallernumber of component parts than otherwise, and which requires a smallerspace than otherwise.

The synchronizing means 116 includes the interlocking rotation shaft117, which operates in conjunction with the second main shaft 16. Thesynchronizing means 116 also includes the first gear-transmissionmechanism 118, which is linked with the first main shaft 15, and whichincreases the speed of the torque transmitted from the first main shaft15. The synchronizing means 116 also includes the secondgear-transmission mechanism 119, which reduces the speed of theabove-mentioned torque. The synchronizing means 116 further includes theswitching mechanism for linkage 120 so as to alternatively switchbetween the states where the interlocking rotation shaft 117 is linkedto the first and the second gear-transmission mechanisms 118 and 119respectively. Providing the synchronizing means 116 between the two mainshafts 16 and 17 makes the synchronizing means 116 smaller, and this, inturn, makes the transmission system M smaller.

The arrangement of the first and the second main shafts 15 and 16, withthe second main shaft 16 coaxially penetrating the first main shaft 15,allows the two shafts 15 and 16 to relatively rotate to each other.Between the engine E and the first end portions of the two main shafts15 and 16, the first and the second hydraulic clutches 28 and 29 areprovided respectively, and the engagement and disengagement of the twoclutches 28 and 29 are alternatively switched. The first and the secondgear-transmission mechanisms 118 and 119, which are placed between thesehydraulic clutches 28 and 29 and the first to the fifth gear trains G1to G5, are linked with the first main shaft 15. The interlockingrotation shaft 117, which has an axis parallel to the first and thesecond main shafts 15 and 16, is linked with, and operates inconjunction with, the second end portion of the second main shaft 16 ina position where the interlocking rotation shaft 117, and the first andthe second gear-transmission mechanisms 118 and 119 are placed atrespective sides of the first to the fifth gear trains G1 to G5.Consequently, the synchronizing means 116 is not placed among the firstto the fifth gear trains G1 to G5. This allows a compact arrangement ofthe first to the fifth gear trains G1 to G5 in the axial direction ofthe two main shafts 15 and 16 and of the countershaft 17, so that thetwo main shafts 15 and 16 as well as the countershaft 17 are madeshorter in the axial direction thereof.

In addition, the actuator 130, which drives the synchronizing means 116,is placed at a side of the clutch system 8, which includes the first andthe second hydraulic clutches 28 and 29. The placement of the actuator130 in an unused space at a side of two hydraulic clutches 28 and 29allows the actuator 130 to be placed closely to the first and the secondgear-transmission mechanism 118 and 119 of the synchronizing means 116,which gear-transmission mechanisms 118 and 119 are placed between thefirst to the fifth gear trains G1 to G5 and the two hydraulic clutches28 and 29. The above arrangement makes the transmission M compact as awhole.

The gear transmission 7 includes the first main shaft 15, which has afirst end portion linked with the first hydraulic clutch 28, and whichis placed in parallel to the crankshaft 9. Also included is the secondmain shaft 16, which has a first end portion linked with the secondhydraulic clutch 29, and which coaxially penetrates the first main shaft15. The first and the second hydraulic passages 76 and 78 areconcentrically formed inside the second main shaft 16, and help applyhydraulic pressure respectively to the first and the second clutchpistons 44 and 54 of the first and the second hydraulic clutches 28 and29. To this end, the operating fluid is supplied from the second endside of the second main shaft 16.

As a result, a large sideward protrusion of the power unit P can beavoided even on the side where the clutch system 8 is located. This, inturn, allows a larger bank angle to be set when the power unit P ismounted on a motorcycle.

The hydraulic control means 100, which controls the supply of theoperating fluid to the first and the second hydraulic passages 76 and78, is placed on the engine cover 63, which forms the oppositeside-surface of the engine E from the clutch system 8. Consequently, thecrankcase 14 is prevented from having a larger vertical dimension aroundthe transmission housing 13, which is formed inside the crankcase 14 toaccommodate the gear transmission 7 therein. Thus, a small crankcase 14is accomplished.

The hydraulic control means 100 is supported on the support plate 105attached to the engine cover 63. A part of at least one of the first andthe second communicating-fluid passages 107 and 108, which respectivelylink the hydraulic control means 100 with the first and the secondhydraulic passages 76 and 78, is formed between the attaching surfacesrespectively of the engine cover 63 and of the support plate 105—in thisembodiment, a part of the first communicating-fluid passage 107 isformed in this way. As a result, the part of the communicating-fluidpassage 107 is formed with ease.

The input member 34 is provided to the clutch system 8. The input member34 includes the annular plate portion 34 a, which receives the powerfrom the engine E, the inner cylindrical portion 34 b, which has thebase end formed integrally with and continuously from the innerperimeter side of the annular plate portion 34 a, and the outercylindrical portion 34 c, which coaxially surrounds the innercylindrical portion 34 b, and which has the base end formed integrallywith and continuously from the outer perimeter side of the annular plate34 a. The inner cylindrical portion 34 b is made to be the input side ofthe first hydraulic clutch 28, and the outer cylindrical portion 34 c ismade to be the input side of the second hydraulic clutch 29. The firstand the second hydraulic clutches 28 and 29 are arranged so that thefirst hydraulic clutch 28 is concentrically surrounded by the secondhydraulic clutch 29. The first and the second clutch pistons 44 and 54,which are provided commonly to and included respectively in the firstand the second hydraulic clutches 28 and 29, are placed on the openingend side of the input member 34.

The above-described structure of the clutch system 8 reduces the numberof component parts of the clutch system 8, and makes the clutch system 8small as a whole. In addition, when the engagement and the disengagementof the two hydraulic clutches 28 and 29 are switched, the output members40 and 50 of the respective hydraulic clutches 28 and 29 engage with acommon input member 34. This results in a smaller change in inertialforce at the time of switching the engagement and disengagement, and ina smooth switching between engagement and disengagement.

Moreover, the first output member 40 of the first hydraulic clutch 28has the first cylindrical portion 40 a coaxially surrounding the innercylindrical portion 34 b, and is linked with the first main shaft 15 soas not to rotate relatively thereto. Meanwhile, the second output member50 of the second hydraulic clutch 29 has the second cylindrical portion50 a coaxially surrounding the first cylindrical portion 40 a inside theouter cylindrical portion 34 c, and is linked with the second main shaft16 so as not to rotate relatively thereto. Consequently, the first andthe second cylindrical portions 40 a and 50 a, which are respectivelyparts of the output members 40 and 50 of the two clutches 28 and 29, areplaced closely to each other. Thus, the inertial mass on the output sideof the first clutch 28 and that of the second clutch 29 get closer toeach other. As a result, no big change in feelings occurs when theclutches engage and disengage.

The first and the second clutch pistons 44 and 54, which are includedrespectively in the first and the second hydraulic clutches 28 and 29,are arranged side by side in the axial direction while the second clutchpiston 54 has a larger diameter than the first clutch piston 44 has.Inside the second main shaft 16, the first and the second hydraulicpassage 76 and 78 are formed to supply the operating fluid to the firstand the second clutch pistons 44 and 54. Consequently, no mechanism forswitching the engagement and disengagement of the two hydraulic clutches28 and 29 is placed outside of the clutch system 8 so that no space forplacing such a mechanism needs to be secured outside of the clutchsystem 8.

The damper spring 12 is set between the annular plate portion 34 a ofthe input member 34 and the driven gear 31 linked to the engine E.Consequently, the fluctuation of the power transmitted from the engine Eto the two hydraulic clutches 28 and 29 is absorbed by the common damperspring 12.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A twin-clutch transmission in a power unit for a vehicle, in whichpower unit the power is transmitted to a gear transmission housed in acrankcase of an engine, via a primary reduction system and a clutchsystem, from an end portion of a crankshaft, wherein the clutch systemincludes a pair of hydraulic clutches respectively having clutchpistons; the gear transmission includes a first main shaft, which has afirst end portion linked with a first one of the hydraulic clutches, andwhich is placed in parallel to the crankshaft, and also includes asecond main shaft, which has a first end portion linked with a secondone of the two hydraulic clutches, and which coaxially penetrates thefirst main shaft; a pair of hydraulic passages are concentrically formedinside the second main shaft to apply hydraulic pressure respectively tothe clutch pistons of the hydraulic clutches and allowing operatingfluid to be supplied from a second end side of the second main shaft. 2.The twin-clutch transmission in a power unit for a vehicle according toclaim 1, further comprising hydraulic control means which controls thesupply of the operating fluid to the two hydraulic passages, wherein thehydraulic control means is placed on an opposite side-surface of theengine with respect to the clutch system.
 3. The twin-clutchtransmission in a power unit for a vehicle according to claim 1, furthercomprising: an engine cover joined with the crankcase; a support plateattached to the engine cover and supporting the hydraulic control means;and a pair of communicating-fluid passages which link the hydrauliccontrol means to the two hydraulic passages, wherein a part of at leastone of the communicating-fluid passages is formed between the attachingsurfaces respectively of the engine cover and the support plate.
 4. Thetwin-clutch transmission in a power unit for a vehicle according toclaim 2, wherein the hydraulic pressure control means includes a firstsolenoid valve and a second solenoid valve, which share a common valvebody.
 5. The twin-clutch transmission in a power unit for a vehicleaccording to claim 3, wherein the hydraulic pressure control meansincludes a first solenoid valve and a second solenoid valve, which sharea common valve body which is fastened to the support plate by bolts. 6.The twin-clutch transmission in a power unit for a vehicle according toclaim 2, wherein an oil filter is attached to an engine cover below thehydraulic pressure control means.
 7. The twin-clutch transmission in apower unit for a vehicle according to claim 1, further comprising aninterlocking rotation shaft below the first and the second main shafts,the interlocking shaft having a length longer than a length of the firstmain shaft and shorter than a length of the second main shaft.
 8. Thetwin-clutch transmission in a power unit for a vehicle according toclaim 3, wherein a groove on an outer surface of the engine cover formspart of the at least one communicating-fluid passage between the enginecover and the support plate
 9. The twin-clutch transmission in a powerunit for a vehicle according to claim 3, wherein a passage hole coveredby a cap is formed in the engine cover in a position that is coaxialwith the first and the second main shafts.
 10. The twin-clutchtransmission in a power unit for a vehicle according to claim 3, whereinthe support plate is fixed to an outer surface of the engine cover in aposition above a cap formed in the engine cover.
 11. A twin-clutchtransmission in a power unit for a vehicle, comprising: a geartransmission housed in a crankcase of an engine for receiving power froma front end of a crankshaft, via a primary reduction system and a clutchsystem, wherein the clutch system includes a pair of hydraulic clutchesrespectively having clutch pistons, and wherein the gear transmissionincludes a first main shaft, which has a first end portion linked with afirst one of the hydraulic clutches, and which is arranged parallel tothe crankshaft, and also includes a second main shaft, which has a firstend portion linked with a second one of the two hydraulic clutches, andwhich coaxially penetrates the first main shaft; and further comprisinga pair of hydraulic passages concentrically formed inside the secondmain shaft to apply hydraulic pressure respectively to the clutchpistons of the hydraulic clutches and allowing operating fluid to besupplied from a second end side of the second main shaft.
 12. Thetwin-clutch transmission in a power unit for a vehicle according toclaim 11, further comprising hydraulic control means which controls thesupply of the operating fluid to the two hydraulic passages, wherein thehydraulic control means is placed on an opposite side-surface of theengine with respect to the clutch system.
 13. The twin-clutchtransmission in a power unit for a vehicle according to claim 11,further comprising: an engine cover joined with the crankcase; a supportplate attached to the engine cover and supporting the hydraulic controlmeans; and a pair of communicating-fluid passages which link thehydraulic control means to the two hydraulic passages, wherein a part ofat least one of the communicating-fluid passages is formed between theattaching surfaces respectively of the engine cover and the supportplate.
 14. The twin-clutch transmission in a power unit for a vehicleaccording to claim 12, wherein the hydraulic pressure control meansincludes a first solenoid valve and a second solenoid valve, which sharea common valve body.
 15. The twin-clutch transmission in a power unitfor a vehicle according to claim 13, wherein the hydraulic pressurecontrol means includes a first solenoid valve and a second solenoidvalve, which share a common valve body which is fastened to the supportplate by bolts.
 16. The twin-clutch transmission in a power unit for avehicle according to claim 12, wherein an oil filter is attached to anengine cover below the hydraulic pressure control means.
 17. Thetwin-clutch transmission in a power unit for a vehicle according toclaim 11, further comprising an interlocking rotation shaft below thefirst and the second main shafts, the interlocking shaft having a lengthlonger than a length of the first main shaft and shorter than a lengthof the second main shaft.
 18. The twin-clutch transmission in a powerunit for a vehicle according to claim 13, wherein a groove on an outersurface of the engine cover forms part of the at least onecommunicating-fluid passage between the engine cover and the supportplate
 19. The twin-clutch transmission in a power unit for a vehicleaccording to claim 13, wherein a passage hole covered by a cap is formedin the engine cover in a position that is coaxial with the first and thesecond main shafts.
 20. The twin-clutch transmission in a power unit fora vehicle according to claim 13, wherein the support plate is fixed toan outer surface of the engine cover in a position above a cap formed inthe engine cover.